A blob of gas spreads out and heats up as it circles the supermassive black hole at the center of the Milky Way galaxy. Known as Sagittarius A-star, the black hole is about 4.2 million times as massive as the Sun and is about 27,000 light-years away. It is normally very "quiet," but it occasionally flares up as material funnels toward it. [ESO/APEX/2MASS/A. Eckart et al./L. Calçada]

The heaviest single object in the entire galaxy is also one of the most difficult to see because it produces no energy at all. But it reveals its presence through its effects on the stars and gas around it.

The object is Sagittarius A-star, a black hole that’s four million times as massive as the Sun. Gravity squeezes it so tightly that nothing can escape it — including light.

The black hole sits at the center of the galaxy, where it’s orbited by stars and gas clouds. Stars move much faster when they’re close to the black hole than when they’re farther away. From that, astronomers calculate the mass of the object the stars are orbiting.

Although the black hole is completely dark, matter around it produces a faint X-ray glow — wisps of gas that are heated as they spiral toward the black hole. And that zone occasionally flares up when an asteroid or other chunk of matter falls into the black hole.

Eventually, astronomers hope to see the black hole itself as a dark outline against the background of stars and gas. Until then, they’ll have to settle for looking at its powerful effects on the center of the galaxy.

And the galaxy’s center is in Sagittarius, which scoots low across the south on summer nights. We can’t see the center with our eyes because it’s hidden behind clouds of dust. It takes special instruments to peer through the dust and behold the wonders in the heart of the Milky Way.

More about those wonders tomorrow.

Script by Damond Benningfield, Copyright 2014

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